Tube amp speaker load question—is it safe to do this?

[Thundersteel]

I built a Mojotone “One Watt Wonder” tube amp recently. It sounds REALLY good for its power level. I also built a Tube Depot 50-watt “British style” amp.  It sounds good, but only when cranked.

What I’d like to do, is feed the output of the 1-watt amp into the input of the 50-watt amp; sort of using it like a pre-amp.

I know both amps require a speaker/appropriate load connected to the outputs. 

Question: If I was to disconnect the signal wire that goes to the power amp tubes in the 1-watter, and then connect it via patch cable to the input of the 50-watter, would I still need to have a speaker connected to the 1-water?

[JGale]

If the one watter has an output transformer and tube output section YES!

Output tubes and tranny need to see a load. It can be a resistor of appropriate size and power but a load nonetheless.

[JGale]

Weber makes what you want. An attenuator with a line out.

https://www.tedweber.com/gadgets/attenuators/

[Thundersteel]

Instead of buying the attenuator, I’m guessing I could probably disconnect the power tube and output transformer from the circuit, correct?

[JGale]

Starting to trespass outside my area of expertise, but I would not disconnect the speaker at all under power unless there is some kind of resistance across the leads.

[JGale]

I used to run my Aurora FXloop send into another amp, but never disconnected the speaker from the power section output. Unless you plan to use the speaker elsewhere, leave it connected, or go to your local electronics store and purchase a 10W-8ohm power resistor as a speaker substitute for the one watt amp. It might get warm.

Whether or not you also need to ground the power amp input will depend on any noise that arises after you disconnect the preamp.

[JGale]

https://grangeramp.com/product/passive-effects-loop-kit/ sells a kit for $8 that has all the parts you need to it right.

Still need resistance across the speaker outs.

Also,

What you like about the one watt amp might be the result of the entire amplification circuit including the output tube and transformer. In that case the Weber or something like it (Bugera) will take that output and swamp it down to line level to feed either the input jack on the larger amp or the larger amp's FXloop Return.

[JGale]

Yeah. I'm back. Damn you for asking about something I enjoy talking about very much.

Kevin O'Conner address your desire in several of his books, but TUT5 contains schematics for what he terms a "Super Scaler".

In short, what is does is take the output of the speaker jack (using speaker cable) into a circuit that looks exactly like the output circuit, but in reverse. A input transformer takes the 8ohm speaker level signal and drops it to a line level signal that powers another entire output section, tube or other.

Or, for those of you on drugs (blood pressure pills count),

Feed the output of the 1-watt transformer into the output side of another, similar, transformer and recover the original signal, including all the tube and tranny interactions so far. Then take that signal and reamplify it to whatever level you build the second output section to support.

QED I hope.

[JGale]

I just noticed a subtlety in your question above. Disconnecting power to the output tubes will increase the voltage to the preamp section since power supply design is a balancing act. London Power, of course, makes a kit, standard in their power scaling models, that controls that.

[Dutchman]

16 hours ago, Thundersteel said:

Instead of buying the attenuator, I’m guessing I could probably disconnect the power tube and output transformer from the circuit, correct?

So you'll just be using the preamp circuit? 

My advise... buy a pedal that does what you want. Or build one with the pre amp circuit in it. My concern is running the high voltages to a 1/4 jack. Preamps for amplifiers see some knock you on your ass voltages. Without tubes!

[JGale]

Well now there's a good point, from an amp builder. Can't have, don't want DC on the signal you tap into and outo which means coupling caps.

Granger also sells a hi-voltage solid state effects loop that will do what you want. If you have enough spare amperage available.

[JGale]

From what I surmise, after a short internet search, is that this is a likely solution to your problems.

https://reverb.com/p/bugera-ps1-power-soak-100-watt-attenuator

I have never used this model. I do have here a very old Weber Mass 15W I use when biasing.

This has a line out, which is what you need to feed your greed, I mean your other amp.

[JGale]

That was the easy solution. It also captures the tone of the amp after the power section adds it's flavor.

For less money and a little sweat equity, you could contact Mojotone and ask them to work you through adding their hi-voltage effects loop to their amp.

https://www.mojotone.com/Mojotone-Discrete-Hi-Voltage-Series-Effects-Loop-for-Vacuum-Tube-Amplifiers

They seem like decent people to me. You already have the skills to implement. You can also add effects to the little amp and additional gain in the return circuit.

 

 

[Thundersteel]

This is what I did: I installed a DPDT switch which cuts the circuit to the output tube filament, and to the output transformer primary. The preamp signal now goes to a shorting-type 1/4" jack.

The result: The 50W amp sounds really good now with its input boosted. I no longer need a distortion pedal in front of it.

Based on previous comments, I may need to tap the preamp signal after the coupling cap instead of before, like it is now. 

[Dutchman]

12 hours ago, Thundersteel said:

Based on previous comments, I may need to tap the preamp signal after the coupling cap instead of before, like it is now

The coupling caps are going to filter out the DC which makes power tubes drunk and angry right before they glow red and die. I'm still not certain what your doing, but remember that your audio has a ~~~ type signal. A Sine Wave, that's what the phase inverter tube does before the power tubes, it gets this signal working correctly. DC has a -------- signal. 

[HSB0531]

On 10/23/2022 at 11:20 PM, Thundersteel said:

Instead of buying the attenuator, I’m guessing I could probably disconnect the power tube and output transformer from the circuit, correct?

Sounds like you want to tap into the preamp output stage of the 1-watt amp and send it to the 50W amp's instrument input.

[Thundersteel]

10 hours ago, HSB0531 said:

Sounds like you want to tap into the preamp output stage of the 1-watt amp and send it to the 50W amp's instrument input.

Exactly!

 

12 hours ago, Dutchman said:

The coupling caps are going to filter out the DC which makes power tubes drunk and angry right before they glow red and die. I'm still not certain what your doing, but remember that your audio has a ~~~ type signal. A Sine Wave, that's what the phase inverter tube does before the power tubes, it gets this signal working correctly. DC has a -------- signal. 

Yes, I know. I've been working in electronics for >30 years, so I think I know what I'm doing! 🤔      I just haven't had years of experience working with tube amps, so thanks for the tips.

Anyway, I moved the preamp tap to after the coupling cap for safety's sake.

 

[HSB0531]

On 11/1/2022 at 11:04 AM, Thundersteel said:

Exactly!

 

Yes, I know. I've been working in electronics for >30 years, so I think I know what I'm doing! 🤔      I just haven't had years of experience working with tube amps, so thanks for the tips.

Anyway, I moved the preamp tap to after the coupling cap for safety's sake.

 

Let us know how it works out and a tone report!